1. Field of the Invention
The present invention relates to a plasma processing apparatus, and more particularly, to a plasma processing apparatus comprising means for generating a rotating magnetic field using a polyphase alternating-current.
2. Description of the Prior Art
Conventionally, a plasma processing apparatus generates a plasma due to an electric discharge between plane-parallel electrodes provided within a processing chamber, and performs processes using the generated plasma such as a process for forming thin films on a surface of a processing substrate, a reforming process, an etching process or the like.
Recently, the following plasma processing apparatus (referred to as a conventional plasma processing apparatus hereinafter) has been suggested in, for example, the Japanese patent laid-open publication No. 61-86942. In the conventional plasma processing apparatus, in order to uniformly process the whole processing surface of a processing substrate in a high speed accompanying larger dimensions of the processing substrates, there are provided a plurality of sets of Helmholtz coils in the periphery of a processing chamber, and a rotating magnetic field is generated at an area close to the processing surface of the processing substrate by symmetrical three-phase alternating-current so as to be parallel to the processing surface thereof.
FIG. 6 schematically shows the conventional plasma processing apparatus.
Referring to FIG. 6, there is provided in a processing chamber 1 having a cylindrical shape, an anode electrode 2 and a cathode electrode 3 which are plane-parallel electrodes, wherein the anode electrode 2 is electrically connected to ground. The cathode electrode 3 is connected through a coupling capacitor 4 to a high frequency power source 5, and a processing substrate 6 is placed on the cathode electrode 3 axially to the processing chamber 1.
In the periphery of the processing chamber 1, there are provided three sets of Helmholtz coils used as means 7 for generating a rotating magnetic field, and each set of Helmholtz coils is comprised of a pair of air-core ring-shaped coils which oppose to each other so that the respective directions of the magnetic fields respectively generated by a pair of air-core ring-shaped coils are parallel to each other.
FIG. 7 shows an arrangement of means 7 for generating the rotating magnetic field comprising three sets of Helmholtz coils 71, 72 and 73.
Referring to FIG. 7, a first set of Helmholtz coils 71 is constituted by a pair of ring-shaped coils 71a and 71b, a second set of Helmholtz coils 72 is constituted by a pair of ring-shaped coils 72a and 72b, and a third set of Helmholtz coils 73 is constituted by a pair of ring-shaped coils 73a and 73b. As shown in FIG. 7, respective sets of Helmholtz coils 71 to 73 are provided so that the positions thereof are rotated by an angle of 120 degrees about the center 6c of the circular-plane-shaped processing substrate, and the central axes f of the coils opposing to each other pass through an area close to the processing surface of the processing substrate 6.
In the above-mentioned Japanese patent laid-open publication No. 61-86942, the winding directions of the Helmholtz coils and the electrical connections between the Helmholtz coils are not shown and also are not described. However, generally, the winding directions of the Helmholtz coils and the electrical connections between the Helmholtz coils are as follows. The winding directions of a pair of coils are the same as each other. Further, for example, the winding completion end of the coil 71a is electrically connected to the winding start end of the coil 71b, and the winding start end of the coil 71a and the winding completion end of the coil 71b are electrically connected to a symmetrical three-phase alternating-current power source for generating a rotating magnetic field. Furthermore, the other coils 72a, 72b, 73a and 73b are electrically connected in manners similar to those as described above.
When processing the processing substrate using the plasma processing apparatus constituted as described above, after a gas within the processing chamber 1 is discharged through an exhaust gas port 1a, a desirable processing gas is introduced through a gas entrance port 1b, and then the pressure of the processing gas within the processing chamber 1 is kept to be a predetermined constant pressure.
Further, a high frequency voltage is applied between the anode electrode 2 and the cathode 3, and simultaneously, respective-phase currents I1, I2 and I3 represented by the following equations (1), (2) and (3) are respectively flowed into the three sets of Helmholtz coils 71, 72 and 73, which are provided so that the spatial angle therebetween becomes 2.pi./3 as described above, from a commercial symmetrical three-phase alternating-current power source for generating a magnetic field. EQU I1=I sin .omega.t (1) EQU I2=I sin (.omega.t-2.pi./3) (2) EQU I3=I sin (.omega.t-4.pi./3) (3)
Then, a circular rotating magnetic field comprised of a combined magnetic field having a magnetic field intensity H represented by the following equation (4) is generated in a clockwise direction about the center 6c of the processing substrate 6. EQU H=(3/2)Hm (4),
where Hm is the maximum value of the magnetic field intensity generated by one set of Helmholtz coils 71, or 73.
As described above, the rotating magnetic field of the combined magnetic field H having the constant magnitude over a wide area is generated in an area close to the processing surface of the processing substrate 6 in parallel to both the electrodes 2 and 3, namely, in parallel to the processing substrate 6, so as to be rotated about the center 6c of the processing substrate 6. Then, a plasma having a high density is uniformly generated, and the processing substrate is processed with a high uniformity in a high speed.
In the above-mentioned conventional apparatus, in order to improve the uniformity of the distribution of the magnetic field in a direction directed to the rim peripheral end of the processing substrate from the center thereof, each of the Helmholtz coils has a size of a window remarkably larger than the diameter of the processing substrate 6. Therefore, it is necessary to provide a large-sized coils, resulting in the large-sized plasma processing apparatus. Further, since each of the Helmholtz coils is an air-core coil in addition to these requirements, the conventional plasma processing apparatus consumes a remarkably large electric power.
Further, as shown in FIG. 7, since the Helmholtz coils are provided so that each of the adjacent coils projects from the window of the coil, it is necessary to manufacture three kinds of coils for respective phases, the sizes of which are different from each other, resulting in a more expensive manufacturing cost.